Carbamate condensation unit

ABSTRACT

A carbamate condensation unit of the submerged type for synthesis urea production plants comprising a tube bundle for the condensation of gaseous compounds, is distinguished in that it further comprises a duct, structurally independent from the tube bundle, for the circulation of part of the condensed gaseous compounds inside the condensation unit.

DESCRIPTION

[0001] 1. Field of Application

[0002] The present invention relates to a carbamate condensation unit ofthe so-called submerged type, used in a plant for the production ofsynthesis urea from gaseous carbon dioxide and liquid ammonia.

[0003] 2. Prior Art

[0004] In order to produce urea, the reactants, i.e. carbon dioxide andammonia, are fed partially condensed in form of carbamate, in asynthesis reactor wherein the condensation of carbamate, an intermediateproduct of the synthesis, is carried out to an almost complete extent.Only a portion of the carbamate is then converted into urea in thereactor itself, by virtue of the chemical balances that characterizethis conversion.

[0005] The remaining portion of unconverted carbamate, together with theunreacted ammonia, is then forced out of the reactor and at leastpartially recovered, by stripping, for example with CO₂, in form ofgaseous ammonia and carbon dioxide by per se known processes.

[0006] These gaseous substances must then be partially condensed, thusobtaining their conversion into liquid carbamate that is then recycledto the synthesis reactor.

[0007] As known, in a plant for urea production, it is required toconvert through condensation into carbamate part of the reactants and ofthe intermediate products that, unconverted into urea in the synthesisreactor, are recovered downstream thereof in form of gaseous ammonia andcarbon dioxide.

[0008] In order to satisfy the aforesaid requirement, in EP-A-1 036 787a condensation unit of the so-called submerged type has been proposed,comprising a cylindrical shell inside which is supported a tube bundle,wherein the tubes are straight and in heat exchange relationship with asuitable coolant.

[0009] In the tube bundle, ammonia and carbon dioxide condensation takesplace, together with their reaction to form carbamate.

[0010] Although advantageous as far as some aspects thereof areconcerned, the condensation unit exhibits a remarkable drawback thatwill be described hereinbelow.

[0011] In fact, only a part, although a major part, of the tubes of thetube bundle are used for the conversion into carbamate of the gaseouscompounds flowing from a lower end to an upper end thereof. Theremaining portion of the tube bundle must instead be used for therecycle of a portion of the condensed gaseous compounds from the upperend to the lower end thereof. The latter in order to allow a naturalcirculation of the liquid phase inside the condenser so as to increasethe heat exchange coefficient of the apparatus.

[0012] Accordingly, given a tube bundle of predetermined size, the yieldis strictly bound to the only part thereof intended for thecondensation.

SUMMARY OF THE INVENTION

[0013] The object of the present invention is to provide a highefficiency condensation unit, wherein, at equal size of the tube bundle,the yield in condensed gaseous compounds is remarkably increased withrespect to the teaching of the prior art.

[0014] This object is achieved by a carbamate condensation unit of thesubmerged type for synthesis urea production plants, comprising:

[0015] a substantially cylindrical shell, closed at the opposed endsthereof by an upper and a lower bottom, respectively, defining anintermediate portion, an upper portion and a lower portion of thecondensation unit;

[0016] a tube bundle for the condensation of gaseous compounds, fittedinto the intermediate portion of the condensation unit and in fluidcommunication with the upper portion and the lower portion;

[0017] characterized in that it comprises:

[0018] a duct, structurally independent from the tube bundle, in fluidcommunication with the upper and lower portions, for the circulation ofpart of the condensed gaseous compounds.

[0019] Preferably, the gaseous compounds NH₃ and CO₂ to be condensedcoming from a stripping unit downstream the synthesis reactor, a flowcomprising carbamate in aqueous solution coming from a urea recoverysection, and optionally a solution comprising urea coming from thesynthesis reactor and feed liquid ammonia, are fed in the lower portionof the condensation unit through respective dedicated openings in thelower bottom.

[0020] The condensed gaseous compounds are instead made exit from thecondensation unit through at least an opening in the upper bottom influid communication with the upper portion of the condensation unit.

[0021] Thanks to the present invention, the whole tube bundle is used inorder to carry out the condensation step. In fact, the presence of aduct structurally independent from the tube bundle, in fluidcommunication with the upper and lower portions of the condensationunit, allows advantageously the circulation of the carbamate outside thetube bundle.

[0022] As the tube bundle is exclusively used to carry out thecondensation, the present invention allows advantageously increasing theyield of such condensation, the size of the tube bundle being the same.

[0023] Further features and advantages of the present invention willappear more clearly from the following non-limiting description of anembodiment thereof, made with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the drawings:

[0025]FIG. 1 shows a schematic view in longitudinal section of acondensation unit obtained according to the present invention.

[0026]FIG. 2 shows a schematic view in longitudinal section of acondensation unit according to an alternative embodiment of the presentinvention;

[0027]FIG. 3 shows schematic view in longitudinal section of acondensation unit according to a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0028] With reference to FIG. 1, with 1 is globally indicated acondensation unit according to the present invention comprising acylindrical shell 2 closed at the opposed ends by an upper bottom 3 anda lower bottom 4, respectively.

[0029] In the shell 2 a tube bundle 5 is supported having apredetermined size, i.e. having a predetermined number of straight tubes6 of predetermined length and diameter, said straight tubes 6 beingsupported by opposed upper and lower tube plates 7, 8, respectively.Said plates 7 and 8 separate the shell 2, which defines an intermediateportion 9 of the condensation unit, from the bottoms 3 and 4, whichdefine an upper portion 10 and a lower portion 11 of the condensationunit, respectively.

[0030] Said portions 10 and 11 are reciprocally in fluid communicationthrough the plurality of straight tubes 6 of the tube bundle 5.

[0031] It shall be noted that said tube bundle 5 is in heat exchangerelationship with a coolant, for example water, flowing outside thetubes 6 (shell side) and fed into the condensation unit and exitingtherefrom through suitable openings and connecting means that are notshown as per se conventional.

[0032] The upper portion 10 of the condensation unit is provided with afirst gas discharge opening 12, formed in the upper bottom 3 and with asecond opening 13 formed in the side part of the portion itself.

[0033] In said portion 10, and in proximity to said second opening 13,an overflow device 14 is provided, which is schematically illustrated bya baffle.

[0034] The lower portion 11 is provided with a first opening 15 to whichpasses a first duct 16 fastened thereto, for feeding the gases to becondensed, as will be clear in the following description.

[0035] Said first duct 16 has a free end 17, located inside a gasdistribution chamber 18, supported in a conventional manner inside saidlower portion 11 of the condensation unit.

[0036] According to the present invention, a duct 19, extending outsidethe shell 2, is in fluid communication on one side with the upperportion 10 of the condensation unit and on the other side with the lowerportion 11 through openings 20 and 21, respectively.

[0037] Said duct 19 is structurally independent from the tube bundle 5and furthermore is in fluid communication with a second feeding duct 22.

[0038] The functional features according to the present invention willbe now indicated by making reference to the figures. In these figures,Fg and Fl generally indicate the flows of the gaseous phase and of theliquid phase inside the condensation unit 1, respectively.

[0039] With reference to FIG. 1, the volume of the condensation unit 1,schematically illustrated when in regular operation, is entirely takenup by an aqueous solution comprising carbamate, ammonia and optionallyurea and by a mixture comprising ammonia, carbon dioxide and water inthe form of vapors.

[0040] Said substances in a vapor phase come from a stripping unit (notshown) downstream a synthesis reactor (not shown) for the decompositionof carbamate and the ammonia and carbon dioxide stripping from the ureasolution coming from the synthesis reactor. These substances are fedinto the condensation unit by the above said first feeding duct 16 anddistributed through the gas distribution chamber 18 inside the lowerportion 11, near a lower end of the tubes 6 of the tube bundle 5.

[0041] This implies a mixing of said gaseous compounds with said aqueoussolution that is already present in the lower portion 11.

[0042] From the lower portion 11, the gaseous compounds and the aqueoussolution mixed together flow inside the straight tubes 6 of the tubebundle 5.

[0043] Inside said tube 6, ammonia, carbon dioxide and water condensateand ammonia reacts with carbon dioxide, thus forming carbamate.

[0044] This carbamate is added to the carbamate already present in theaqueous solution inside the condensation unit 1, obtaining in this way,at the outlet of the tubes 6, one carbamate solution possibly comprisingalso urea.

[0045] The carbamate solution flows in the upper portion 10, wherein afirst part thereof is recycled to the lower portion 11 through the duct19, and a second part thereof exits the unit 1 through the opening 13with a flow regulated by the overflow device 14. The portion of solutionexiting the unit 1 through the opening 13 is then sent to the synthesisreactor for the conversion into urea of the carbamate and ammoniatherein contained.

[0046] It shall be noted that the duct 19 provides the circulation ofthe aqueous solution inside the condensation unit 1, in particular fromthe upper portion 10 of the condensation unit to its lower portion 11.

[0047] Said circulation guarantees that the straight tubes 6 are alwaysfull of solution and contain a constant amount thereof. Further on, itallows an optimal crossing speed through the tubes 6 to be maintained bythe liquid phase to all advantage of the heat exchange between saidliquid phase and the coolant flowing outside the tubes 6, and thereforeof a more effective condensation of the gaseous compounds.

[0048] A part of the aqueous solution flowing inside the condensationunit 1 is supplied from outside through a flow comprising carbamate inaqueous solution coming from a urea recovery section (not shown) andpossibly unreacted substances coming from the synthesis reactor and feedliquid ammonia. Said flow is fed to the lower portion 11 of the shell 2through a duct 22 that ends up into the duct 19 in order to be added tothe recycle aqueous solution. In this way no further openings shall beprovided into the lower bottom 4 of the condensation unit 1.

[0049] Once introduced in the lower portion 11 of the condensation unit1, the aqueous solution mixes up with the gaseous compounds coming outof the distribution chamber 18 and as described above is madecirculating within the condensation unit 1.

[0050] Should any gaseous substance be still present in the upperportion 10, they will be vented from the condensation unit 1 through theopening 12 obtained in the upper bottom 3.

[0051] The synthesis reactor, the stripping unit and the condensationunit 1 are all part of the so-called high-pressure synthesis loop ofplant for the industrial production of urea. Such apparatuses do in factoperate substantially at the same pressure and are connected the one tothe other in order to make possible the separation and recycle to thesynthesis reactor of at least a portion of the unreacted substancescontained in the urea solution coming out therefrom.

[0052] The above described condensation unit 1 is subject tomodifications and changes.

[0053] Thus, an alternative embodiment of the invention is for exampleshown with reference to FIG. 2.

[0054] In this figure, the details of the condensation unit 1 that arestructurally and functionally equivalent to those illustrated in FIG. 1will be indicated with the same reference numbers and will not bedescribed any more.

[0055] The condensation unit 1 comprises a tank 27 arranged externallyto the upper bottom 3.

[0056] The tank 27 comprises at least a first lower opening 33 (two inthe example of FIG. 2), a second lower opening 32 in fluid communicationwith the duct 19, a third gas discharge opening 28 in the upper portionand, on a side, a fourth opening 29.

[0057] In turn, the upper bottom 3 of the condensation unit 1 has atleast an upper opening 34 to which a nozzle 31 is associated (two in theexample of FIG. 2).

[0058] In FIG. 2, the tank 27 is in fluid communication with the upperportion 10 of the unit 1 through the lower openings 33 fixed to thecorresponding nozzles 31 of the upper bottom 3.

[0059] In proximity to the fourth opening 29 an overflow device 30 isprovided, which is schematically illustrated by a baffle.

[0060] The functional characteristics according to the presentalternative embodiment of the invention will be now described withreference to FIG. 2.

[0061] According to this embodiment, the aqueous solution flowing fromthe tube bundle 5 to the upper portion 10 is fed into the tank 27through the openings 34, the nozzles 31 and the openings 33.

[0062] A first portion of the aqueous solution collected inside the tank27 is recycled to the lower portion 11 of the unit 1 through the opening32 and then the duct 19, whereas a second portion comes out of thecondensation unit 1 through the opening 29 to be fed to the ureasynthesis reactor. The overflow device 30 regulates the outlet flow fromthe opening 29.

[0063] Finally, should any gaseous substance be still present in thetank 27, it will be vented from unit 1 through the opening 28.

[0064] This alternative embodiment is particularly advantageous for therevamping of pre-existing condensation units, for example condensationunits of the film type.

[0065] In said units, the liquid phase is made flow for gravity insidethe tubes of a tube bundle as a film of liquid in co-current with thegaseous compounds to be condensed.

[0066] In the preexisting condensation units it is not possible neithereconomically convenient to make structural modifications to the same, inparticular to the shell.

[0067] Advantageously, thanks to this embodiment of the invention, atank 27 is provided to set in fluid communication the upper portion 10of the condensation unit 1 with the lower portion 11 thereof, for thecirculation of the aqueous solution inside this unit without the need ofintervening onto the existing structure of the shell 2 and of thebottoms 3 and 4.

[0068] With reference to FIG. 3, a further embodiment of the inventionis shown.

[0069] In this figure, the details of the condensation unit 1 that arestructurally and functionally equivalent to those illustrated in FIG. 1will be indicated with the same reference numbers and will not bedescribed any more.

[0070] As an alternative to the external duct 19 represented in theprevious figures, this embodiment provides the arrangement of a duct 23,structurally independent from the tube bundle 5, which extendsinternally to the condensation unit 1 through the shell 2 and issupported by the tube plates 7 and 8. The duct 23 is in fluidcommunication with the upper and lower portions 10 and 11 throughrespective upper and lower ends 25, 26 for the circulation of thecondensed gaseous compounds within the condensation unit 1.

[0071] In the example of FIG. 3, the duct 23 is arranged coaxially tothe shell 2 and is extended lengthwise from the upper portion 10 nearthe baffle 14 to the lower portion 11 in proximity to the lower bottom4.

[0072] A second opening in the lower bottom 4 of the condensation unit 1is indicated with 24. This opening is for the feed into the lowerportion 11 of the condensation unit 1 of the flow comprising carbamatein aqueous solution coming from the urea recovery section and optionallya solution comprising urea coming from the synthesis reactor and feedliquid ammonia.

[0073] According to this embodiment, beside the yield increase in thecondensation unit 1 with respect to the prior art, an advantageousreduction of the overall volume of the condensation unit itself isachieved. Further on, the costs for manufacturing such a unit are lowerthan those for manufacturing the units according to the previouslydescribed embodiments of the invention, because of the absence of anexternal high-pressure duct.

[0074] In fact, the duct 23 is advantageously arranged within thecondensation unit 1 in order to carry out the circulation of the aqueoussolution inside it.

1. Carbamate condensation unit (1) of the submerged type for synthesisurea production plants, comprising: a substantially cylindrical shell(2), closed at the opposed ends thereof by an upper (3) and a lower (4)bottom, respectively, defining an intermediate portion (9), an upperportion (10) and a lower portion (11) of the condensation unit; a tubebundle (5) for the condensation of gaseous compounds, fitted into saidintermediate portion (9) of the condensation unit, in fluidcommunication with said upper portion (10) and said lower portion (11)and comprising an upper (7) and a lower (8) tube plate, respectively,and a plurality of tubes (6) supported by them; a duct (16) extended insaid lower portion (11) below said tube bundle (5) for feeding saidgaseous compounds to be condensed; discharge openings (12, 13) providedin said upper portion (10) for discharging outside the condensation unituncondensed gaseous compounds and condensed gaseous compounds,respectively; characterized in that it comprises: a duct (19),structurally independent from said tube bundle (5) and said dischargeopenings (12, 13), in fluid communication with said upper and lowerportions (10, 11) for the circulation of part of said condensed gaseouscompounds.
 2. Condensation unit (1) according to claim 1, characterizedin that said duct (19) is external to said shell (2).
 3. Condensationunit (1) according to claim 2, characterized in that it furthercomprises a tank (27) associated to said upper bottom (3) and in fluidcommunication with said upper portion (10) and said duct (19) externalto said shell (2), said discharge openings (28, 29) being provided insaid tank (27).
 4. Carbamate condensation unit (1) according to claim 1,characterized in that said duct (23) extends internally to saidcondensation unit and is supported by said tube plates (7, 8). 5.Carbamate condensation unit (1) according to claim 4, characterized inthat said duct (23) is arranged coaxially to said shell (2).
 6. Methodof retrofitting a pre-existing carbamate condensation unit (1) forsynthesis urea production plants, comprising! a substantiallycylindrical shell (2), closed at the opposed ends thereof by an upper(3) and a lower (4) bottom, respectively, defining an intermediateportion (9), an upper portion (10) and a lower portion (11) of thecondensation unit; a tube bundle (5) for the condensation of gaseouscompounds, fitted into said intermediate portion (9) of the condensationunit, in fluid communication with said upper portion (10) and said lowerportion (11) and comprising an upper (7) and a lower (8) tube plate,respectively, and a plurality of tubes (6) supported by them, saidmethod being characterized in that it comprises the steps of: providinga duct (19), structurally independent from said tube bundle (5) andexternal to said shell (2), in fluid communication with said upper andlower portions (10, 11) for the circulation of part of said condensedgaseous compounds; and providing a tank (27) associated to said upperbottom (3), in fluid communication with said upper portion (10) and saidduct (19) external to said shell (2) and comprising discharge openings(28, 29) for discharging outside the condensation unit uncondensedgaseous compounds and condensed gaseous compounds, respectively;providing a duct (16) extended in said lower portion (11) below saidtube bundle (5) for feeding said gaseous compounds to be condensed.